1. Field of the Invention
The present invention relates to a portable telephone system such as a personal handyphone system (PHS), and more particularly, to the improvement of a speech communication and a data communication in a time division multiple access (TDMA) portable telephone system.
2. Description of the Related Art
A typical TDMA portable telephone system is a PHS.
A first prior art PHS including a base station and a mobile station (PHS terminal) carries out a communication by one time slot assigned thereto. That is, when a service is requested from the PHS terminal through a control channel to the base station, the base station determines a protocol for the service, so that either a speech communication or a conventional data communication is carried out by using an assigned time slot.
In the above-described first prior art PHS, however, since only one time slot is assigned to a speech communication or a conventional data communication, it is impossible for the PHS terminal to simultaneously carry out a speech communication and a conventional data communication. As a result, when a speech communication is requested while a conventional data communication is being carried out, this speech communication is rejected. On the other hand, when a conventional data communication is requested while a speech communication is being carried out, this conventional data communication is rejected.
A second prior art PHS carries out a speech communication and a conventional data communication by two time slots simultaneously (see: JP-A-2000-13858). That is, when a first service is requested from the PHS terminal through the control channel to the base station, the base station determines a protocol for the first service, so that the first service is carried out by using one assigned time slot. Next, while the first service is being carried out, when a second service is requested from the PHS terminal through the control channel to the base station, the base station determines a protocol for the second service, so that the second service is carried out by another assigned time slot.
In the above-described second prior art PHS, however, since a plurality of time slots are assigned to one PHS terminal, effective use is not made of time slots, i.e., channels. In other words, a large number of channels are consumed, so that other communication for the base station would be interrupted.